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Remediated water expands irrigation options

Jan 15, 2009

Irrigating with recycled water is one potential solution to water scarcity, according to research conducted by Agricultural Research Service (ARS) scientists Jean McLain and Clinton Williams. The scientists are examining the effects of irrigating with "reclaimed" water — sewage water that has been treated to remove contaminants.

Preliminary results suggest that reclaimed water is both safe and effective for irrigation of public land in arid regions like Maricopa, Ariz., home of the U.S. Arid-Land Agricultural Research Center, where McLain and Williams are based.

Since September 2006, the two researchers have collected soil and water samples from a municipal park that is being irrigated with reclaimed water. They have tested the samples for dangerous Escherichia coli pathogens. To date, the team has not found a single pathogenic strain of E. coli. The scientists did note a small increase in soil salinity, but not enough to harm plant growth.

Williams has also tested samples for carbamazepine, an epilepsy drug that has been detected in trace amounts in drinking water. His research has shown that natural organic matter found in soil can prevent carbamazepine from leaching beyond the root zone.

McLain and Williams have also addressed the accuracy of the tests used to confirm remediated water's safety, an important step towards gaining the public's approval.

In one study, the researchers established that the quality of reclaimed water is harder to assess in winter, when Environmental Protection Agency-approved assessment methods return more false positives for E. coli. Over several months, they drew samples from a wetland fed with reclaimed water, and tested them for E. coli. The scientists placed the samples on culture media that change color when E. coli colonies develop on them.

In December and January, they observed a surprising increase in positive tests. However, genetic assays revealed that about 90 percent of these were false positives. So why was the accepted culture method delivering such inaccurate results?

Chemical analysis revealed that the water's salt content increased in the winter months, suggesting that the false results may be related to salt chemistry. The ARS researchers are currently collaborating with scientists at the University of Arizona to confirm this hypothesis, which would further support the conclusion that reclaimed water is safe for irrigation.